外部リンク自動検出のやつ source:rskull

gistfile1.js

(function () {
	var regex = /^(https?:\/\/(?:[0-9a-z\-]\.?)+(?::\d+)?)(?:\/.*)?$/,
		replace = "$1",
		baseURL = location.href.replace(regex, replace),
		links = document.links,
		i, len, href;
	
	for (i = 0, len = links.length; i < len; i ++) {
		href = links[i].href;
		if (regex.test(href) && href.replace(regex, replace) !== baseURL) {
			links[i].target = "_blank";
		}
	}
})();